1. Field of the Invention
The invention relates to a method of operating an internal combustion engine, more specifically a diesel combustion engine.
2. The Prior Art
Combustion processes for diesel combustion engines with substantially homogeneous combustion—what are termed alternative diesel combustion processes—make it possible to drastically reduce engine emissions. Specifically, concurrent reduction of NOx and of particles in the engine exhaust is thereby possible. These new combustion processes rely on homogenization of the in-cylinder charge prior to the combustion event. Diesel combustion engines with homogeneous combustion are known from the printed documents U.S. Pat. No. 5,832,880 A, U.S. Pat. No. 6,260,520 B1, U.S. Pat. No. 6,276,334 B1 or from U.S. Pat. No. 6,286,482 B1.
As contrasted with conventional combustion processes, it can be observed in alternative combustion processes that the engine exhausts (NOx, particles, HC, CO and noise) are much more sensitive to the engine operation parameters (injection timing, exhaust gas recirculation rate, fresh air temperature, temperature of the intake manifold, pressure in the intake manifold, exhaust back pressure, coolant temperature, atmospheric pressure). In reverse, changing quite slightly, by a few percent, the exhaust gas recirculation rate suffices to considerably change the NOx emissions for example. In this context, FIG. 1 shows the influence of the exhaust gas recirculation rate and of injection timing on the NOx emission of the engine during alternative combustion. As can be seen from FIG. 2, injection timing and exhaust gas recirculation rate also have considerable influence on particle emission. A slight change in injection timing suffices to heavily influence particle emission.
This fact is at the origin of the need for exactly complying with the engine operation parameters required for alternative combustion processes in order to be able to tap the full potential of alternative diesel combustion processes. With the currently utilized methods for computing certain engine operation parameters (such as injection timing and desired value for the exhaust gas recirculation rate) the control within the engine control system is a simple control as a function of engine speed and engine load, that is to say there is no so-called “closed loop” control. For conventional combustion processes, which have far less sensitiveness between the engine operation parameters and the resulting engine emissions, this simple control is sufficient. Using alternative combustion processes for diesel engines, these control processes however are insufficient because of the sensitiveness described so that the search for new methods continues. The reason therefor is that in the simple control calculation of certain engine operation parameters such as injection timing and exhaust gas recirculation rate currently used, the influence of engine speed, engine load, fresh air temperature, atmospheric pressure and coolant temperature is only taken into consideration statically in characteristic diagrams or lines within the engine control system.
When operating a diesel engine with alternative combustion together with the currently used control strategy, two critical operating conditions occur. Firstly, if the exhaust gas recirculation rate is too high, combustion becomes instable. The 50% mass fraction burned is too near top dead center, which results in incomplete combustion with high emissions (HC and CO) and in an instable engine torque. Secondly, if the exhaust gas recirculation rate is too low, the 50% mass fraction burned is advanced, this involving considerable increase of combustion noise.
A system for controlling the exhaust gas recirculation rate in a compression ignition internal combustion engine is known from DE 31 34 631 A1, in which a desired value for ignition delay is determined and the actual value of ignition delay is controlled to match this desired value. The desired value for ignition delay thereby originates from an engine characteristic map. Ignition delay time is obtained comparing the signals, for example the start of injection by an injection nozzle and of a pressure sensor connected to the combustion chamber.
An automatic control for a self-igniting internal combustion engine in which the in-cylinder peak pressure is measured and compared with a desired value is known from GB 2 091 000 A. The control variable changed as a result of this difference is injection timing.
Both in DE 31 34 631 A1 and in GB 2 091 000 A, only one control variable is changed. This is not sufficient for controlling a diesel combustion engine with homogeneous combustion.
The most important variables for determining the combustion process in an internal combustion engine are the phasing of the combustion process or of start of combustion, the maximum speed of pressure increase in the cylinder, and the peak pressure.
In an internal combustion engine in which combustion substantially occurs through self-ignition of a directly injected quantity of fuel, the determining variables are mainly determined by injection timing, charge composition and ignition delay. These parameters are in turn determined by a great number of influencing variables such as speed, fuel quantity, intake temperature, boost pressure, effective compression ratio, inert gas content of the in-cylinder charge and component temperature.
Conventional diesel combustion essentially is a diffusion process in which air and fuel are not mixed together but separately delivered to the combustion zone. Conventional diesel combustion is characterized by the inhomogeneous distribution of air and fuel. The concentration of the fuel in the injection spray decreases continuously from the inside to the outside toward the region of the surrounding air-residual gas mixture. Combustion in zones at air conditions within the range of stoichiometric air ratio and below leads to high peak temperatures resulting in thermal NOx formation. Further, lack of oxygen in rich zones combined with high temperatures results in the formation of soot.
A more stringent legal framework makes it necessary to always find new ways for designing combustion processes in order to reduce emission of soot particles and NOx emission in diesel combustion engines.
It is known to reduce NOx and soot emission in the exhaust by increasing ignition delay, advancing therefor the ignition timing so that combustion occurs through self-ignition of a lean fuel-air mixture. A possible variant thereof is termed HCLI process (Homogeneous Charge Late Injection). When such mixture combustion is carried out, fuel injection occurs sufficiently far from the top dead center of the compression period, so that a largely homogeneous fuel-air mixture is obtained. Exhaust gas recirculation permits to keep combustion temperature below the minimum temperature needed for NOx to be generated. Since homogenization of fuel and air is time-dependent, the realization of this process is restricted, being dependent both on speed and on charge, as particle emission increases if homogenization is insufficient.
U.S. Pat. No. 6,338,245 B1 describes a diesel combustion engine relying for operation on the HCLI process in which combustion temperature and ignition delay are adjusted so that at lower and medium part load the combustion temperature is lower than the NOx formation temperature and the air ratio is greater than the value that is relevant for soot formation. The combustion temperature is thereby controlled by changing the exhaust gas recirculation rate and ignition delay, by fuel injection timing. At medium and high load, the combustion temperature is lowered to such an extent that the formation of both NOx and soot is avoided. The disadvantage thereof is that, at medium part load particularly, a low air ratio occurs together with low combustion temperatures with poor efficiency trade-off.
U.S. Pat. No. 6,158,413 A describes a direct injection diesel combustion engine in which fuel injection is not set to take place before compression top dead center and in which oxygen concentration in the combustion chamber is minimized through exhaust recirculation. This method of operation is also termed the HPLI process (Highly Premixed Late Injection). Due to the temperature level that decreases after top dead center—as compared to a conventional injection before top dead center—and to the increased quantity of recirculated exhaust over conventional operation, ignition delay is longer than in what is termed diffusion combustion. The low temperature level controlled by the exhaust gas recirculation rate causes the combustion temperature to remain below the value relevant for NOx formation. The long ignition delay effected by the later ignition time permits to obtain a good blend so that, as a result thereof, the local lack of oxygen during combustion of the mixture is significantly reduced and the formation of particles is decreased. Retarding the combustion process results in a lower maximum temperature but at the same time in a higher mean temperature at a given late crank angle so that the burning off of soot is enhanced. Moreover, causing combustion to occur in the expansion stroke together with the high exhaust gas recirculation rate leads, in spite of the larger quantity of pre-mixed fuel due to the long ignition delay and, as a result thereof, in spite of the higher maximum combustion rate, to an in-cylinder pressure increase rate that does not exceed the admissible value. The disadvantage thereof is the poor efficiency in the lower part load range.
The Austrian Utility Model Application GM 702/2002 suggests operating a diesel combustion engine in the lower part load range in the HCLI mode, in the medium part load range in the HPLI mode and in the full load range with conventional diesel combustion. As a result, the internal combustion engine can be operated with high efficiency and low NOx and soot emissions in any load range.
The HCLI process and the HPLI process pertain to the alternative diesel combustion processes.
It is known to determine injection timing for the fuel on the basis of engine operation parameters or through the control of characteristic diagrams. It is further known to compute injection timing through a combustion regulator with feedback on the actual combustion situation. For stationary condition, injection timings determined in this manner are sufficient.
In dynamic operation of the engine though, transiently occurring differences in the in-cylinder charge as compared to the stationary desired values result in a difference between the resulting combustion noise and the stationary desired values.
A method for controlling an internal combustion engine is known from DE 43 22 319 C2 in which a first actual value is prescribed starting from a value λ and said first actual value and a first desired value are prescribed by a first control means, starting from a first control variable. Further, a second actual value can be prescribed from a quantity of air and, starting from said second actual value and a second desired value, a second control variable is prescribed by a second control means. The desired values are thereby chosen so that, if certain operating conditions are given, the desired values are prescribed for the quantity of air, and if these certain operating conditions are not given, desired values are prescribed for the value λ.
It is known to determine start of injection or combustion situation in an internal combustion engine using for example an in-cylinder pressure sensor and to obtain therefrom control signals for controlling the internal combustion engine such as injection timing. DE 197 49 817 A1 suggests computing the start of injection and the combustion situation from the measured pressure history and from the calculated pressure history.
It is the object of the invention to control combustion in a diesel combustion engine with homogeneous combustion in the simplest possible way and with the greatest possible accuracy. It is another object to develop a method by means of which the internal combustion engine can be operated in the optimal mode at each operating point. It is still another object of the invention to propose a method of operating an internal combustion engine by means of which, in dynamic operation of the engine, the combustion noise can, as far as practicable, be kept at the values of the stationary engine operation.